Of frogs and men: the origins of psychophysiological time experiments, 1850–1865 Henning Schmidgen Towards the end of the 1840s, Hermann von Helmholtz began to investigate experimentally the propagation of stimuli within nerves. Helmholtz’s experiments on animals and human subjects opened a research field that in the following decades was intensively explored by neurophysiologists and experimental psychologists. Helmholtz’s pioneering investigations justify the central place he occupies in accounts of the history of modern psychophysiology. Studying the concrete experimental settings and their local contexts shows how deeply the work of scholars such as Helmholtz is embedded in the history of culture and technology. In particular, the rapidly growing technologies of electromagnetism, which gave rise to telegraphy and electric clocks, facilitated the time measurements of 19th-century physiologists and psychologists. On 15 January 1850, Hermann von Helmholtz (1821–1894) technology and the military were closely interrelated. signed a short report and sent it off to the Physical Three years before publishing the first volume of his Society and the Academy of Sciences in Berlin. Its first path-breaking Investigations on Animal Electricity paragraph read: (1848), Emil Du Bois-Reymond (a friend and colleague of Helmholtz) had made concrete suggestions for meas- I have found that a measurable time passes when the stimu- uring ‘the speed of the muscle and nerve activity’ at a lus exerted by a momentary electric current on the hip plexus meeting of the Physical Society3. Then in 1847, the jour- (Hüftgeflecht) of a frog propagates itself to the nerves of the nal of this society, Advances of Physics, printed the report thigh and enters the calf muscle. In large frogs whose nerves of a young lieutenant in the Prussian artillery. Under the were 50 to 60 millimetres long, and which I had kept at title ‘On speed measurements’, Werner Siemens, who 2–6 degrees Celsius (whereas the temperature of the obser- later founded the well-known electricity group, offered an vation room was between 11 and 15 degrees), this length of overview on the most recent methods for exactly deter- time amounted to 0.0014 and 0.0020 of one second.1 mining the ‘speed of bodies’. Almost all of the devices Siemens presented and explained relied on the application of electromagnetism. And whether it was Charles Helmholtz and the velocity of nervous Wheatstone’s chronoscope or the apparatus devised by stimulation Siemens himself, the main field for using these apparatus Ever since the formation of the reflex concept in the 17th was military research. As Siemens put it, especially century, naturalists had been interested in the speed of the ‘to the artillery it is of importance to know the speed ‘animal spirits’ and the conduction velocity of the ‘nerv- of projectiles at various points of their trajectory.’4 ous principle’. With his investigations, Helmholtz entered Furthermore, the military background of ‘time the history of the experimental life sciences as the first microscopy’ was not unfamiliar to Hemholtz. Between scientist to make precise measurements of nervous action. 1843 and 1848 he had received his training as squadron In nerve-muscle preparations from the frog, Helmholtz surgeon and military physician in Potsdam5. measured a propagation velocity of stimuli between 25 When measuring the propagation velocity of stimula- and 43 meters per second2. tions in nerves, Helmholtz initially used a method sug- Helmholtz’s experiments were carried out in a world gested by Claude Pouillet (1790–1868). As the French in which physiology (or ‘organic physics’), telegraph physician had reasoned, the degree of deflection of a galvanometer needle depended not only on the intensity Henning Schmidgen of the electric current, but also on the time during which Is Research Scholar at the Max Planck Institute for the History the current affected the needle. It thus should also have of Science in Berlin. His research focuses on the modern been possible to deduce the ‘current time’ from the history of experimental physiology and psychology. He also deflection of the needle. Pouillet devised an electric cir- has a strong interest in bringing the history of the cuit in which electric impulses of temporal precision experimental life sciences to the Internet (see, in particular, could be produced by means of a rotating disk on which http://vlp.mpiwg-berlin.mpg.de). Currently, he is writing a book about the history of physiological and psychological circumscribed contact surfaces were mounted. Given a time experiments. constant electric current, the deflection of a galvonometer e-mail: [email protected] needle included in the circuit could be used to derive the 142 Endeavour Vol. 26(4) 2002 0160-9327/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S0160-9327(02)01448-5 Figure 1. In the 19th century, most of the procedures to measure short time intervals were developed within the context of military research. Their primary use was measuring the velocity of projectiles at different points of their trajectories. With the advent of telegraph technology in the late-1830s, electromagnetism began to play an important Figure 2. This contemporary reconstruction shows the use role in this kind of precision measurement. The Figure shows Helmholtz made of Pouillet‘s method for measuring the the use of such a time measuring device in the context of propagation speed of electric stimuli in the nerve. The set-up ballistics. Its basic mechanism was suggested by the French combined two electric circuits: one in which the battery, R, by physicist Claude Pouillet in 1844. The deflection of a passing through the induction spirales I1 and I2, could galvanometer needle served to measure the time during momentarily stimulate the nerve muscle preparation N/M; the which the current was active. In this set-up, firing the rifle other circuit connected the galvanometer T to the battery Z. opened the current which was closed again by the bullet The switch S/P allowed closure of both electric circuits at cutting the wire at the rifle‘s muzzle. In the late-1840s, exactly the same time. The devices within the metallic frame Hermann von Helmholtz adopted Pouillet‘s method for A guaranteed that after the contraction of the frog muscle the measuring the propagation speed in the nerves. Reproduced time-measuring circuit was permanently interrupted. By from Ref. 30. placing the electrode at different points of the nerve (n1, n2), Helmholtz could deduce the time the nerve needed for the time during which the current was active. Pouillet com- conduction of the stimulus. Reproduced from Ref. 31. pared this procedure to the ballistic pendulum Benjamin Robbins had constructed some hundred years previously6. as ‘nerve and muscle physics’ and, in more recent times, No wonder then that contemporary handbooks of electro- has been labeled ‘neurophysiology’. Scholars such as magnetism displayed figures illustrating the use of Gabriel Gustav Valentin and Albert von Bezold, Pouillet’s method for ballistic purposes (Figure 1). Julius Bernstein and Étienne Jules Marey, and Keith In his experiments on the propagation of nerve stimula- Lucas and Edgar Douglas Adrian contributed significant- tions, Helmholtz adopted Pouillet’s ‘means to measure ly to this field. Less widely known is the fact that extremely short time intervals’ for physiological use. Helmholtz was interested in the time relations structured Helmholtz devised two interconnected electric circuits, by the nervous systems of living beings not just from a one to stimulate the nerve-muscle preparation, the other physiological but also from a psychological point of to measure the current time by means of a galvanometer. view8. In fact, at the time at which he performed his stud- Both circuits could be closed in such a way that the prepa- ies on frogs, Helmholtz carried out similar experiments in ration was electrically stimulated and the electric current human beings. Again, Helmholtz’s aim was to measure was simultaneously sent through the galvanometer. When the stimulation propagation in the sensory nerves. the muscle then started contracting, the current in the In comparison with his frog experiments, Helmholtz’s galvanometer circuit was immediately and permanently work on humans presented special challenges. For obvi- interrupted. From the deflection of the needle, Helmholtz ous reasons, the experiments could not be conducted on could read off the time the muscle needed to contract after preservable preparations separated from the rest of the stimulation. Placing the electrode at different parts of the organism. Helmholtz tried to circumvent this difficulty by nerve-muscle preparation, the differences in the results measuring what today’s psychologists would call ‘simple obtained allowed him to calculate the propagation speed reaction times’. In a communication to the Physical of the ‘nervous impulse’ (Figure 2). Society signed by Helmholtz on 15 December 1850, he This, however, was only the initial form of Helmholtz’s reported on ‘experiments on the propagation speed of experiments. In the extended investigations he carried out stimuli in the sensory nerves of human beings’, and after his first report, Helmholtz adopted other methods for described the basic structure of his experiments: his time measurements, turning to graphical procedures. But when devising his famous ‘frog myographion’, he In a human being, a very weak electric shock is applied to a again relied on existing precision technologies – this limited space of skin. When he feels the shock, he is asked time, the indicator tools used by steam engineers7. to carry out a specific movement with the hand or the teeth, interrupting the time measurement as soon as possible.9 Helmholtz’s psychological experiments Amongst historians of biology, it is understood that When all parameters remained constant, Helmholtz found Helmholtz’s work on the propagation speed of nervous in the results of these experiments a ‘surprising constan- stimuli constitutes the beginning of a branch of research cy’.